The Spanish Slug (Arion lusitanicus). A highly weedy species, and a likely survivor of a future mass extinction.

This is probably one of the single best scientific essays I’ve ever read. David Quammen has interviewed paleontologist David Jablonski of University of Chicago on the cycles of mass extionction on the earth, and the scenario which is likely to procede the next one. The essay begins with the premise of rapid extinction -species are going extinct now at a rate matched by only a few instances in the earth’s history.

This elevates the perspective, and our place in the earth’s history, to the epic scope. As some of you already know, extinction and speciation come and go in cycles. The earth has seen at least five Extinction Level Events (ELE’s) – cataclysmic events caused by natural catastrophes, where a large part of the contemporary wildlife is eradicated. Those we know of now are the Ordovician, Devonian, Permian, Triassic and Cretaceous ELE’s. The last one, usually referred to as the K-T event, is also the most well-known: the one which proved the demise of the dinosaurs 65 million years ago, and opened the way for mammals to fill the ecologic niches left by them.

So, most indicators say we’ve got prime seats for the next ELE scenario – actually, we’re in the middle of it right now. The rate at which species are going extinct right now is only matched in the fossil record by these cataclysmic mass extinction.

So then, if we look ahead a little – how is the earth likely to look, say, a hundred years down the same road – with mankind only increasing in numbers, and both animals and plats going extinct? Which wildlife will survive this human-induced ELE?

The answer may surprise you, and you don’t have to look very far to find them. Noticed the fly you just tried to swat? The rats you’re putting out poison to stomp out? The doves that crap all over town?

Say hello to your new neighbors. These are the kind of animals we’re likely to be left alone with.

This demands some explanation. Now, some species just seem extremely successful and hardy – able to withstand the most organized attempts at extinction. We often refer to them as pests, because they sucessfully compete with us and prey at human resources. Jablonski calls them weedy species, in the same sense as we talk of weed in the garden. So what is it that defines them? Quammen elaborates:

“What do fire ants, zebra mussels, Asian gypsy moths, tamarisk trees, maleleuca trees, kudzu, Mediterranean fruit flies, boll weevils and water hyacinths have in common with crab-eating macaques or Nile perch? They’re weedy species, in the sense that animals as well as plants can be weedy. What that implies is a constellation of characteristics: They reproduce quickly, disperse widely when given a chance, tolerate a fairly broad range of habitat conditions, take hold in strange places, succeed especially in disturbed ecosystems, and resist eradication once they’re established. They are scrappers, generalists, opportunists. They tend to thrive in human-dominated terrain because in crucial ways they resemble Homo Sapiens: aggressive, versatile, prolific, and ready to travel.”

These are the species who are likely to survive a mass extinction. They’re way more “extinction-proof” than the endemic, rare and highly specialized (say, dependent on a very specific climate condition or food source) species we tend to value.

Thus, these weedy species are the ones we’re likely to be left alone with on earth.
As much as we hate them, these are our true competitors, and the true champions of nature – volatile, hardy and widespread survivors. The aga toads and wild rabbits (both introduced by humans) will roam the Australian countryside freely, only disturbed by the occasional pack of feral dog – and certainly without any trace of the long-vanished marsupial fauna which once competed with them. Insecticide-resistant lineages of the much-hated Spanish slug and Colorado beetle will continue to plague gardens and crop fields all over Europe. The occasional patch of rainforest, where once ape eagles, birds of paradise and parrots flew, will be filled with crows and house sparrows – the ground dominated by the hardy rodent cosmopolites brown rat, pacific rat and house mouse.

Insects? Tons, and tons of them – although not the spectacular, beautiful brands we see on nature programmes. I’m afraid you’ve already guessed what will come of the house fly and banana fly…

And of course, there will be bacteria and virii. Only seeing the omnipresent macrofauna and spectacular megafauna, we tend to forget that the arcetypical organism on earth still is single-celled, and often bacterial. They were here first, and they’re likely to be the one to leave last – long after the earth is uninhabitable for any multi-celled organism.

So, will this bleak vision of pests, weeds and patches of green be the end of life on earth? Is this what it will be like for the rest of our planet’s history? Have we humans signed the death warrant of life?

Of course not. That would be to overestimate the roles of humans. We are not the crown of any creation, nor the teleological goal of evolution. We are but a very successfull branch on the tree of life, the one among a number of hominid species and sub-species which actually made it to world domination – in a way which is unprecedented in the former history of the planet. But that doesn’t mean that life stops and starts with us.

The earth may look like a rather desolate and washed-out place when our menace is over – lacking in biological diversity, and only filled with weedy, generalistic species. But do remember – this is what happens in every extinction.

Remember what happened in the K/T-event, which snuffed out the dinosaurs. The only species which did survive were the weedy generalists: the small, rodent-like mammals, the birds, and a few others. All the diverse and specialized fauna of the earth hail from these hardy survivors.

In the same way, the future fauna of earth will decend from the generalists who survive us: the rats, the pidgeons, the house fly. They will provide the basis for future speciation, when the ELE of human rampage has either come to a complete end through our demise, or gone through a population bottleneck so severe we cannot afflict the same amount of damage on nature.

It takes a while for evolution to “branch out” after an ELE – Jablonski estimates 5-10 million years in general. Then, we will be back at a scenario quite similar to before humans rose to prominence on earth. Quammen concludes:

“Still, evolution never rests. It’s happening right now, in weed patches all over the planet. I’m not presuming to alert you to the end of the world, the end of evolution, or the end of nature. What I’ve tried to describe here is not an absolute end but a very deep dip, a repeat point within a long, violent cycle. Species die, species arise. The relative pace of those two processes is what matters. Even rats and cockroaches are capable–given the requisite conditions; namely, habitat diversity and time–of speciation. And speciation brings new diversity. So we might reasonably imagine an Earth upon which, 10 million years after the extinction (or, alteratively, the drastic transformation) of Homo sapiens, wondrous forests are again filled with wondrous beasts. That’s the good news.”.